The present invention relates to a frequency modulator utilizing a frequency synthesizer, which is applied to a mobile radio communication apparatus.
A conventional frequency synthesizer for a frequency modulator of this kind comprises a voltage controlled oscillator (VCO), a programmable frequency divider, a reference oscillator, a phase detector (comparator) and a phase locked loop connecting them. With this circuit configuration, an input modulation signal from an external source is applied to a variable capacitance diode (variable capacitor) contained in the VCO to generate a frequency-modulated signal. Particularly, for a mobile radio communication use, an output frequency of the VCO is changed according to control of a channel selection and change of the VCO output frequency is performed by changing a frequency-dividing factor of the programmable frequency divider.
When such frequency synthesizer is applied to a mobile radio communication apparatus, it is well known in the field that modulation sensitivity is undesirably deviated according to the channel selection, i.e., the VCO output frequency. For example, when, in order to generate a higher carrier frequency, a control voltage to be applied to the VCO is boosted, the modulation sensitivity becomes higher. Namely, since the control voltage applied to the VCO corresponds to a reversed voltage applied to the variable capacitor in the VCO, capacitance of the variable capacitor becomes reduced when the control voltage becomes high. Therefore, capacitance change due to the level of the input modulation signal becomes more efficient than that due to the control voltage.
Conventionally, the modulation sensitivity change is compensated by controlling a level of an input modulation signal according to channel selection. Such technique is disclosed in U.S. Pat. No. 4,501,019 "FREQUENCY MODULATION TRANSMITTER" issued on Feb. 19, 1985 and assigned to the applicant of the present invention.
However, according the forementioned conventional technique, there is the following disadvantage. When an oscillation frequency band becomes wider, i.e., the number of channels used for mobile radio communication becomes increased, it is necessary to enlarge the circuit configuration for compensating the corresponding wide modulation sensitivity change. Therefore, a frequency synthesizer becomes more complicated.